C# solution for Leetcode question: Convert BST to Greater Tree (difficulty : easy)

Source:

Given a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus sum of all keys greater than the original key in BST.

Example:

Input: The root of a Binary Search Tree like this:
5
/ \
2 13
Output: The root of a Greater Tree like this:
18
/ \
20 13

My C# solution is here:

https://github.com/jiajionline/LeetcodeSolutionWithMultipleLanguages/tree/master/Convert%20BST%20to%20Greater%20Tree